Working wavelength of photonic devices, such as semiconductor lasers and modulators, is determined by an optical bandgap of a semiconductor heterostructure having a quantum well structure, a multiple quantum well structure, a superlattice structure or a quantum dot structure. Other opto-electronic components, such as waveguides and optical interconnects, need to operate at an optical frequency that is non-resonant with the bandgap. To achieve monolithic integration of opto-electronic and photonic devices, one should be able to selectively modify bandgap across a wafer.
Inter-diffusion of atoms in column III or column V or both in the periodic table of elements across a heterojunction of a heterostructure (in short, called as quantum well intermixing, or QW intermixing) has been widely used to post-growth-tune a bandgap. Several methods are well known in the prior art to enhance inter-diffusion.
One method is referred to as impurity-induced QW intermixing and has been demonstrated in a variety of heterostructures by using a diffusion process. See a review article by Marsh, in “Quantum Well Intermixing”, Semiconductor Science and Technology, vol. 8, 1993, pp. 1136–1155. This method suffers from several drawbacks. The presence of doping impurities changes conductivity and conductive type, which either deteriorates or completely kills the device performance. Introduction of neutral impurities like F and B by ion-implantation generates traps and residual damages, which also deteriorate the device performance.
The second method is referred to as ion-implantation induced QW intermixing. Ion-implantation generates point defects, such as vacancies in places remote from or over an active region. A method for QW intermixing by implanting ions directly into an active region and then subjecting the structure to thermal annealing suffers from the fact that a high temperature post-annealing may not fully recover from crystal damages caused by ion-implantation and may introduce inhomogeneous QW intermixing. (See Hirayama et al. in “Ion-Species Dependence of Inter-diffusion in Ion-Implanted GaAs-AlAs Superlattices”, Japanese Journal of Applied Physics, vol. 24, 1985, pp. 1498–1502). Elman et al. in U.S. Pat. No. 5,238,868 disclosed a method for QW intermixing, in which vacancies and defects generated by ion-implantation are spatially separated from an active region, and in post-annealing, redistribution of those vacancies and defects enhances QW intermixing. This type of methods involves multiple ion-implantation and thermal annealing (see Charbonneau et al. in U.S. Pat. No. 5,395,793). Re-growth is required in some cases where a top ion-implantation damaged region needs to be removed after QW intermixing (See Paquette et al. in “Blueshifting of InGaAsP/InP laser diodes by low energy ion implantation”, Applied Physics Letters, vol. 71, 1997, pp. 3749–3751). A large dose of ion-implantation required for a large post-growth tuning often degrades the quality of the heterostructure. (See, Tan et al. in “Wavelength shifting in GaAs quantum well lasers by proton irradiation”, in Applied Physics Letters, vol. 71, 1997, pp. 2680–2682).
A third method is commonly referred to as impurity-free vacancy-enhanced intermixing (IFVEI) of QWs. IFVEI has been extensively investigated since its initial report by Deppe et al. in Applied Physics Letters, vol. 49, 1986, pp. 510–512. In IFVEI, a dielectric layer is deposited on the top-surface of the heterostructure. At an elevated post-annealing temperature, atomic vacancies of elements in column III, column V or both in the periodic table of elements, such as Ga, or P vacancies, are generated at an interface between the dielectric layer and the top-surface of the heterostructure. A subsequent diffusion of these vacancies into a heterojunction of the heterostructure enhances inter-diffusion of atoms in column III or column V or both in the periodic table of elements across the heterojunction or in other words, enhances QW intermixing. Comparing the above-mentioned methods, the effect of IFVEI on degradation of electrical and optical properties is minimal, which is especially advantageous if intermixed regions are to be used as an active region or part of an active region in a device. IFVEI usually uses a PECVD (plasma enhanced chemical vapor deposition) deposited or e-beam evaporated SiO2 or spin-on silica as a dielectric layer. The use of a spin-on silica film as a dielectric layer has shown several advantages over the others. IFVEI as a function of annealing conditions has been well described in the prior art, but the difficulty in spatial selection of IFVEI still remains, particularly in the case where more than two different optical bandgaps are needed in close proximity on a wafer.
An approach for spatially selective IFVEI was the use of SrF2 as a layer to inhibit QW intermixing. When the SrF2 coverage varies from 0% to 50%, by varying the space between 1 mm stripes of SrF2, the wavelength shift at 77K varies from 20 nm to 5 nm in a non-linear way after 30s annealing at 925° C. (See Ooi et al. in IEEE Journal of Quantum Electronics, vol. 33, 1997, pp. 1784–1793). This method suffers from a drawback that in order to allow uniform intermixing at a QW depth by overlapping vacancy diffusion fronts, the dimension of SrF2 masks has to be smaller than or comparable to diffusion lengths of point defects. Electron beam lithography is usually employed to generate SrF2 features of sub-micron to one-micron size. Moreover, a SrF2 mask also induces damages and may crack due to thermal stress at an elevated post-growth annealing temperature. The method is difficult to use under manufacturing conditions, and in giving reproducible results.
Cohen et al. (Applied Physics Letters 73:803–805, 1998) described how point defects can be engineered when epitaxial layers covered by a GaAs oxide layer are annealed at a high temperature. This oxide covered quantum wells was found to have interdiffusion of an order of magnitude higher than uncovered layers. When a thin layer of Al was evaporated over the oxide layer prior to rapid thermal annealing, the rate of interdiffusion was found to be reduced by more than an order of magnitude compared to uncovered ones. It was found that QW interdiffusion can be either increased or decreased during high-temperature processing by manipulation of the point defect concentrations.